An electrical potential difference between two electrodes located on opposite surfaces of an elastomeric body generates an electric field leading to a force of attraction. As a result, the distance between the electrodes changes and the change leads to compression of the elastomeric material which is thereby deformed. Due to certain similarities with a muscle, an elastomer actuator is sometimes referred to as an artificial muscle.
U.S. Pat. No. 6,376,971 discloses a compliant electrode which is positioned in contact with a polymer in such a way, that when applying a potential difference across the electrodes, the electric field arising between the electrodes contracts the electrodes against each other, thereby deflecting the polymer. Since the electrodes are of a substantially rigid material, they must be made textured in order to make them compliant.
The electrodes are described as having an ‘in the plane’ or ‘out of the plane’ compliance. In U.S. Pat. No. 6,376,971 the out of the plane compliant electrodes may be provided by stretching a polymer more than it will normally be able to stretch during actuation and a layer of stiff material is deposited on the stretched polymer surface. For example, the stiff material may be a polymer that is cured while the electroactive polymer is stretched. After curing, the electroactive polymer is relaxed and the structure buckles to provide a textured surface. The thickness of the stiff material may be altered to provide texturing on any scale, including submicrometer levels. Alternatively, textured surfaces may be produced by reactive ion etching (RIE). By way of example, RIE may be performed on a pre-strained polymer comprising silicon with an RIE gas comprising 90 percent carbon tetrafluoride and 10 percent oxygen to form a surface with wave troughs and crests of 4 to 5 micrometers in depth. As another alternative, the electrodes may be adhered to a surface of the polymer. Electrodes adhering to the polymer are preferably compliant and conform to the changing shape of the polymer. Textured electrodes may provide compliance in more than one direction. A rough textured electrode may provide compliance in orthogonal planar directions.
Also in U.S. Pat. No. 6,376,971 there is disclosed a planar compliant electrode being structured and providing one-directional compliance, where metal traces are patterned in parallel lines over a charge distribution layer, both of which cover an active area of a polymer. The metal traces and charge distribution layer are applied to opposite surfaces of the polymer. The charge distribution layer facilitates distribution of charge between metal traces and is compliant. As a result, the structured electrode allows deflection in a compliant direction perpendicular to the parallel metal traces. In general, the charge distribution layer has a conductance greater than the electroactive polymer but less than the metal traces.
The polymer may be pre-strained in one or more directions. Pre-strain may be achieved by mechanically stretching a polymer in one or more directions and fixing it to one or more solid members (e.g., rigid plates) while strained. Another technique for maintaining pre-strain includes the use of one or more stiffeners. The stiffeners are long rigid structures placed on a polymer while it is in a pre-strained state, e.g. while it is stretched. The stiffeners maintain the pre-strain along their axis. The stiffeners may be arranged in parallel or according to other configurations in order to achieve directional compliance of the transducer.
Compliant electrodes disclosed in U.S. Pat. No. 6,376,971 may comprise conductive grease, such as carbon grease or silver grease, providing compliance in multiple directions, or the electrodes may comprise carbon fibrils, carbon nanotubes, mixtures of ionically conductive materials or colloidal suspensions. Colloidal suspensions contain submicrometer sized particles, such as graphite, silver and gold, in a liquid vehicle.
The polymer may be a commercially available product such as a commercially available acrylic elastomer film. It may be a film produced by casting, dipping, spin coating or spraying.
Textured electrodes known in the prior art may, alternatively, be patterned photolithographically. In this case, a photoresist is deposited on a pre-strained polymer and patterned using a mask. Plasma etching may remove portions of the electroactive polymer not protected by the mask in a desired pattern. The mask may be subsequently removed by a suitable wet etch. The active surfaces of the polymer may then be covered with the thin layer of gold deposited by sputtering, for example.
Producing electroactive polymers, and in particular rolled actuators, using the technique described in U.S. Pat. No. 6,376,971 and U.S. Pat. No. 6,891,317 has the disadvantage that direction of compliance of the corrugated electrodes is very difficult to control.
Finally, in order to obtain the necessary compliance using the prior art technology, it is necessary to use materials having a relatively high electrical resistance for the electrodes. Since a rolled actuator with a large number of windings will implicitly have very long electrodes, the total electrical resistance for the electrodes will be very high. The response time for an actuator of this kind is given by τ=R·C, where R is the total electrical resistance of the electrodes and C is the capacitance of the composite. Thus, a high total electrical resistance results in a very long response time for the actuator. Thus, in order to obtain an acceptable response time, the number of windings must be limited, and thereby the actuation force is also limited, i.e. response time and actuation force must be balanced when the actuator is designed.